Sherlock fans could be looking at an 18 month wait for their next fix of the detective drama – and even then they might get just one episode rather than the usual three...

A period of filming in early 2015 "looks pretty likely" according to Martin Freeman – who plays Dr John Watson in the BBC1 series – and will initially result in a one-off Christmas special.

“If that [episode] is going to be a special – I’m speaking off-message here; if this was New Labour I’d get fired – I think that might be for next Christmas," Freeman told The Telegraph.

"A Christmas special. That’s what I understand.”

But what form could a festive episode take? Freeman is staying tight-lipped about that at least, but two possibilities immediately present themselves.

First, the next adventure could follow on without a break from the series three cliffhanger finale which was set at Christmas and appeared to show the return from the dead of Sherlock's arch enemy Moriarty, played by Andrew Scott.

But if writers Mark Gatiss and Steven Moffat feel like keeping viewers in suspense for even longer – or, indeed, tying up the loose end and moving on – it could be a stand-alone adventure, perhaps based on the original Sir Arthur Conan Doyle story The Blue Carbuncle, a mystery involving a stolen gem and a Christmas goose.

Sherlock co-creator Mark Gatiss has made no secret of his affection for the tale – he once told RadioTimes.com "I love, love, love Blue Carbuncle!" – and although he and fellow writer Steven Moffat tend to blend elements from Doyle's stories, rather than dramatising them wholesale, it seems likely they would want to pay tribute to it in some form.

Freeman's revelation could delight and dismay Sherlock fans in equal measure. On one hand, many have long been clamouring for a Christmas episode (particularly one based on The Blue Carbuncle); on the other they were hoping to avoid another two-year wait between the last series, shown in January, and the next instalment – and would have been expecting a full three-part run rather than a single episode.

But getting Freeman and his co-star Benedict Cumberbatch together to shoot more Sherlock has proved increasingly difficult, with both now major Hollywood players with packed diaries. Gatiss, meanwhile, has his fingers in numerous acting and scripting pies, including as a writer on Doctor Who, of which Moffat is showrunner.

Nevertheless, Gatiss told RadioTimes.com recently that a full fourth series of Sherlock is still "the plan", which suggests that any Christmas episode would be an extra present for fans...

A BBC spokesperson was unavailable to respond to Freeman's comments.